North Korean technicians monitor the launch of the Unha-3 satellite rocket in 2012 from the Sohae launch centre where an ICBM is believed to be undergoing engine tests. Photograph: Xinhua/KCNA/Corbis

New satellite imagery indicates North Korea has been testing the engine for an intercontinental ballistic missile, a US thinktank has said, amid concerns the North is preparing a nuclear test.

The US-Korea Institute at Johns Hopkins University said images of the North's main rocket launch site suggested one "and maybe more" tests of what was probably the first stage of a KN-08 road-mobile ICBM.

With this latest activity, three KN-08 rocket engine tests have been identified for the first and possibly second stages of the missile dating back to mid-2013, the institute said on its website 38 North.

"As this effort progresses the next technically logical step in the missile's development would be a flight test of the entire system," it said.

North Korea successfully put a satellite into orbit in December 2012 on one of its Unha rockets. Pyongyang said the mission was purely scientific but the international community said the launch was a disguised ballistic missile test and the UN security council tightened sanctions as a result.

The Unha-3 stood 30 metres high and 38 North on Thursday said satellite images showed the gantry at the Sohae launch site was being modified to take larger rockets of up to 50 metres in height.

"Construction may not be completed until early summer, effectively preventing a launch from the facility in the meantime," it said.

The signs of engine testing come amid concerns that the North is on the verge of carrying out a fourth nuclear test in the face of stern international opposition. Satellite imagery of the North's main Punggye-ri nuclear test site has shown increased activity consistent with preparations for a test.

Missile delivery has often been cited as the main weakness of the North's nuclear weapons programme which, after three tests, is thought to be getting closer to mastering the key technology of warhead miniaturisation.

The successful 2012 satellite launch caused serious concern but experts stressed that it lacked the re-entry technology needed to bring an ICBM down on to a target.

Models of the road-mobile KN-08 missile were given pride of place in North Korean military parades in 2012 and 2013. But several experts ridiculed the models, with at least one respected aerospace engineer labelling them technically preposterous and a "big hoax". But developing a reliable, working ICBM remains a priority for the regime and a successful test of such a missile would take the perceived nuclear threat posed by the North to a new level.

On 1 May every year, thousands of North Koreans take part in official celebrations for International Workers Day. But recently, festivities have exposed the rift between the haves and have-nots, writes Daily NK